Typically, a solid oxide fuel cell generator comprised a plurality of tubular solid fuel cells that react a gaseous fuel, such as reformed natural gas, with air to produce electrical power and a hot exhaust gas. Previously, it has been proposed to integrate such a solid oxide fuel cell generator with a gas turbine, where electrical power is produced by both the solid oxide fuel cell generator and the turbine. Such a system was used with a topping combustor supplied with a second stream of fuel to provide a still further heated hot gas that was then expanded in a turbine, as taught in U.S. Pat. No. 5,413,879 (Domeracki et al.).
There, the system is a pressurized-SOFC-generator/gas turbine (PSOFC/GT) hybrid system. It gets high efficiency (relative to the efficiency of a conventional SOFC power system) because it recovers SOFC exhaust heat and converts a fraction of that heat to electric power, and because it operates the SOFC generator at elevated pressure, which boosts cell voltage, which means the SOFC generator runs at a higher efficiency. However, such a system is both complex and expensive.
A wide variety of integrated SOFC/gas turbine systems have been proposed, in, for example, Proceedings of the Power-Gen International '96, “Solid Oxide Fuel Cell/Gas Turbine Power Plant Cycles and Performance Estimates”, Wayne L. Lundberg, Dec. 4-6, 1996 Orlando Fla.; and U.S. Pat. No. 5,573,867 (Zafred et al.). A variety of integrated designs have also been used in molten carbonate fuel cell technology, for example, U.S. Pat. Nos. 3,972,731 and 4,622,275 (Bloomfield et al. and Noguchi et al. respectively).
More recently Stephen E. Veyo and Wayne L. Lundberg et al. in the Proceedings of ASME Turbo Expo 2003, “Tubular SOFC Hybrid Power System Status”, Jun. 16-19, 2003, Atlanta Ga., described current design atmospheric-pressure SOFC/gas turbine (“ASOFC/GT”) hybrid cycle systems, as well as a turbocharged SOFC hybrid cycle, among others. There, in the ASOFC/GT design (shown in FIG. 1) cycle air is taken in at a gas turbine compressor, and preheated with SOFC exhaust heat recovered at a single recuperator. This reduces the gas turbine combustor fuel requirement to achieve a prescribed turbine inlet temperature (TIT), and raises the system cycle electric efficiency. The oxidant for the SOFC module is the turbine exhaust, which will typically be at a pressure that is approximately 1-3 psi above the atmospheric pressure. Thus, an advantage of the system is that the module will not require the complication and expense of design for pressurization, and a module with features from a conventional atmospheric-pressure SOFC power system could be employed.
Systems based on the ASOFC/GT cycle will not be limited to a particular electric power capacity, and it is expected that capacities ranging from circa 100 kwe's to multi-MWe's will be feasible. Further, electric efficiencies (net AC/LHV) of approximately 52% are expected from ASOFC/GT systems. The system could also incorporate a heat export feature, giving it combined heat and power capabilities. For an ASOFC/GT system configured as shown in the article (and FIG. 1), the gas temperature at the turbine expander exhaust must be the oxidant temperature that is required at the SOFC module inlet, and for this to occur, the GT pressure ratio and TIT at GT rating will therefore be restricted to combinations that will result in the required module inlet temperature, or the GT must be operated off-design to achieve the required module inlet temperature. This could limit the number of commercially-available gas turbines that are suited for deployment in a ASOFC/GT system of specified capacity, and if GT off-design operation were required to use a particular GT, reduced system power and efficiency performance could result. Thus, a design issue with power systems that are based on the basic ASOFC/GT cycle is that it is difficult to go out and buy a gas turbine that will provide exactly the air temperature and air flow rate combination that the SOFC generator needs at its inlet to keep its cells running at the right temperature.
What is needed is a modification to the basic ASOFC/GT cycle that would enable the application of gas turbines that did not operate with the preferred expander exhaust temperature under rating conditions, and it could preclude the need to operate the GT off-design for expander exhaust temperature control purposes. It is one of the main objects of this invention to provide a modification to the system that would facilitate easy control of the oxidant temperature between the gas turbine and an associated SOFC module. There is a need to allow the gas turbine in ASOFC/GT made to be less dependent on SOFC module operational requirements.